As a representative steel wire for cold forging in the prior art, a spheroidized annealed material and non-heat treatment steel are publicly known. The spheroidized annealed material is one which may be forged in a cold temperature by spheroidizing carbides precipitated during a heat treatment process to increase ductility. That is, the carbides which are precipitated by heating at a temperature of about 700° C. over 13 to 17 hours is spheroidized, such that its tensile strength lowers to 500-600 MPa to facilitate the forging of the same. However, it has drawbacks in that after a forging process, an additional heat treatment must follow to increase the tensile strength, leading to difficulty in the process and an increase of production cost.
Hence, recently non-heat treatment steel has been briskly developed by omitting the heat treatment of quenching and tempering after a forging process so as to attain cost reduction. Most non-heat treatment steels are composite structures of a ferrite and a pearlite and added by numerous alloy elements, so that they have no problems to forge a part with relatively simple and low machinability having a tensile strength less than 800 MPa. However, in reality, cold forging of a part with complicated shapes having a tensile strength more than 900 MPa, such as a hexagonal flange bolt or the like, is impossible because a cementite present in the pearlite as a lamellar shape is early fractured. Meanwhile, in consideration of the problems mentioned in the conventional steel wire for cold forging, the applicants of the present invention have developed a steel wire, as a high strength steel wire for cold forging with a new concept which does not create cracks during cold forging of a part such as a bolt or the like despite a very high tensile strength of 700-1,300 MPa, which is registered as Korean Patent No. 046971. Recently, in accordance with continuing pursuit of global environmental preservation and development of a light-weight vehicle by automakers there has been raised a strong need to develop a super high strength steel wire having a range of 1,200-1,600 MPa and capable of being cold forged. However, the tensile strength with such a range as in the above is too high, which easily generates cracks during a forging process and considerably lowers the service life of a mold for cold forging. Therefore, in fact, it is impossible to further increase the strength of the steel wire with present technology levels.